The homeostasis of the erythron is accomplished to a major degree by the hormone, erythropoietin, which regulates marrow activity according to tissue requirements for oxygen. This program is investigating the physiology of the bone marrow's response to erythropoietin in order to clarify the quantitative relationship between the level of erythropoietin production, and its concentration in serum and urine. Erythropoietin bioassay techniques will be employed in the detection of subnormal, as well as normal, levels of erythropoietin production in man. Attempts will be made to establish a rational basis for the use of androgenic steroids in stimulating erythropoiesis in certain hematologic diseases in man.